After only a somewhat recent introduction, optical fiber has had a meteoric rise as the predominant means of transmission media in voice and data communications. Optical fiber is manufactured by drawing the fiber from a preform which is made by any of several well known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool to provide a package. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example.
Also, it is common to use an optical fiber package in operations such as ribboning, cabling, and rewinding and to ship optical fiber to other companies which further process the fiber. The optical fiber typically is used in voice and data communications systems, both commercial and military. For example, the package may be used in weapons systems in which it is used for guidance and for data communications. Such uses include communications lines between a projectile, such as a missile, and a control station at a launch site, for example. Optical fiber provides the advantages of increased data bandwidth, reduced weight and greater range than wire-guided systems of the prior art.
A typical optical fiber application in a weapons system involves the packaging of a continuous length of optical fiber on a bobbin which is positioned inside a vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at a launch site. During and after launch, two-way communications with the vehicle are conducted. Such a vehicle commonly is referred to as a tethered vehicle.
In order to use such an arrangement, there must be provided a stable package of the optical fiber which may be disposed within the vehicle and which will permit reliable deployment of the optical fiber during the flight of the vehicle. An adhesive material disposed on surface of the convolutions must provide tack which is sufficiently low to permit payout without causing extreme bends at peel-off points. On the other hand, not enough tack may result in failure through dynamic instability on the bobbin. With respect to optical performance, optical attenuation at the peel-off point may occur through localized macrobending, degrading the integrity of data and video transmission.
There are disadvantages, not present in other forms of communication, in using optical fiber, particularly in a tethered vehicle application. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends in the fiber which are generated by bending or by other stresses to which the fiber may be subjected. Microbending in the layers of undeployed fiber on the bobbin during deployment can affect adversely optical performance. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal. Likewise, physical or optical integrity may be affected adversely by any sharp bends which are experienced as the fiber is deployed from its packaged configuration.
Optical fiber wound on such a bobbin must be measured for expected bend loss due to small radii bends occurring at the peel-off point during high speed deployment. Although in actual deployment, each point along the outer surface of a length of fiber is potentially subjected to momentary bending at the peel-off point, it is now customary in the art to measure such loss only at end points of the fiber package. In a typical bending loss test, fiber is wound around a mandrel of a predetermined radius. Measurements are made prior to and subsequent to the bending of the fiber to determine the loss.
Bending loss measurements such as those just described are not totally acceptable for optical fiber wound on a spool such as one which may be used in tethered vehicles. What is desired is a technique for measuring bend-induced loss from the peel point along the entire length of an optical fiber with minimal contact of the optical fiber.
What seemingly is not included in the prior art are methods and apparatus for measuring bend-induced loss along a length of optical fiber instead of confining such measurement to end portions of the fiber. Of course, the sought after methods and apparatus should not be expensive nor unwieldy to implement. Desirably, such measurements could be carried out with commercially available optical fiber test equipment.